[SCTP]: Use the {DEFINE|REF}_PROTO_INUSE infrastructure

Trivial patch to make "sctcp,sctpv6" protocols uses the fast "inuse
sockets" infrastructure

Each protocol use then a static percpu var, instead of a dynamic one.
This saves some ram and some cpu cycles

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>